PIP: A comprehensive review of what was known of the epidemiology of breast cancer as of the early 1970s was published by MacMahon et al. in 1973. This review covers the major aspects of the epidemiology of breast cancer included in the 1973 review and, additionally, emphasizes recently reported work. Attention is directed to the following: magnitude of the problem in the U.S.; demographic characteristics of breast cancer cases; international variation; laterality of breast cancer; reproductive variables; benign breast diseases; multiple primary cancers involving the breast and other sites; familial aggregation and genetics; endogenous hormones (estrogens, progesterone, prolactin, androgens, and thyroid); exogenous estrogens; diet; body build; radiation; exposure to radiation in screening for breast cancer; mammographic parenchymal patterns; viruses; other exposures of current interest (reserpine, hair dyes); and breast cancer in males. The high incidence and mortality rates and the detrimental impact on the quality of life of those affected indicate that breast cancer in the U.S. continues to be a serious problem for women. An annual age-adjusted incidence rate of 84.9/100,000 women was reported for the 1973-1976 years; the annual age-adjusted mortality rate among women in the U.S. was 27.7/100,000 over this same time period. From these figures, it may be estimated that each year in the U.S. almost 100,000 cases of breast cancer are diagnosed, and over 30,000 deaths occur. Age specific incidence rates increase rapidly with age until about 45-50 years of age, after which they continue to increase but at a slower rate. In addition to age, a few other risk factors, including a history of bilateral premenopausal breast cancer in a 1st degree relative, a history of breast cancer in the contralateral breast, and residence from an early age in North America compared to Asia, are associated with large relative risks. Other risk factors, including whether or not an oophorectomy has been performed, age at 1st birth, a history of fibrocystic disease, previous exposure to high levels of radiation in the chest, socioeconomic status, obesity, and a previous cancer in the ovary or endometrium are associated with relative risks of at least 2 but less than 4-fold. Finally, age at menarche, age at menopause, marital status, place of residence, and the white compared to the black race are associated with small but real differentials in risk.